1. Field of the Invention
The present invention relates to an automatic picture size control method which can automatically control the picture size in a semiwide-screen television receiver wherein the horizontal effective picture size can be widened and thus right and left corner portions of the picture, which are invisible due to overscanning, can be viewed on a display screen, providing a horizontally extended picture on the screen.
Also, the present invention relates to an apparatus for detecting inmost boundaries of right and left edge portions of the screen where the image signal is displayed.
2. Description of the Prior Art
Generally, television broadcasting stations transmit video signals having a 12.times.9 or a 4.times.3 aspect ratio (AR). Referring to FIG. 1, the overall number of scanning lines in the NTSC system is 525, while the number of scanning lines for the effective image (i.e., overall scanning lines--scanning lines for the vertical blanking periods: 525H-2=20H=485H, where 1H is a period for a scanning line) is 485. Also, the effective horizontal scanning period is determined by subtracting the horizontal blanking period from one scanning period (1H) (63.5 .mu.s-10.9 .mu.s=52.6 .mu.s).
However, in a typical television receiver of the NTSC standard, 100% of an image signal is not displayed on the screen of a cathode ray tube (CRT).
FIG. 2 illustrates test patterns shown on the screen for testing the image received in a television receiver. FIG. 3 illustrates a test pattern shown on the screen for testing an overscanning among the test patterns of FIG. 2.
In a typical NTSC type television receiver, an overscanning of about 9.4% in vertical and horizontal directions is performed, and thus the vertical and horizontal edge portions of 9.4% of the 4.times.3 AR image signal transmitted from the broadcasting station cannot be viewed. The reason why such an overscanning is performed in a television receiver is to prevent a distorted image from being displayed on the edge portions of the screen due to an unstable deflection, unstable focusing, unstable high voltage, etc. of the CRT. Specifically, the inductance dispersion in a deflection unit causes the instability of the image displayed on the edge portions of the screen. That is, if the inductance of the horizontal deflection coil is large in comparison with a normal value, the corresponding impedance is increased and horizontal deflection current is decreased, causing the horizontal width of the image to become smaller and thus causing a part of the blanking interval to be viewed on the right and left edge portions of the screen. On the contrary, if the inductance is small, the corresponding impedance is decreased and horizontal deflection current is increased, causing the horizontal width of the image to become larger and thus causing picture information being concealed at the edges of the screen to be increased.
In order to satisfy the limited conditions of the television receiver as described above and to display a stable image on the screen, the overscanning of 9.4% in vertical and horizontal directions has been adopted by manufacturers of television receivers since 1960 when the television receivers were in commercial use.
Recently, a wide-vision television receiver which displays a video signal having a 12.times.9 AR on a screen having a 16.times.9 AR has been introduced. According to this wide-vision, the scanning lines are not changed, but the image displayed on the screen is horizontally expanded by about 1.33 times. Meanwhile, techniques for receiving a 16.times.9 AR video signal transmitted from a broadcasting station and displaying the received 16.times.9 AR video signal on a display screen of a 4.times.3 AR or of a 16.times.9 AR have been introduced. For instance, U.S. Pat. No. 5,386,236 discloses such a technique.
However, according to the wide-vision television receiver, the image displayed on the 16.times.9 AR screen is distorted since the image signal having the 4.times.3 AR is merely expanded in a horizontal direction. Further, since the image is compressed in the vertical or horizontal direction, black panels are shown on the top and bottom portions, or the right and left edge portions of the screen, deteriorating the availability of the screen.
In order to solve the problems involved in the prior art, the inventor filed a United States patent application for the invention entitled "A semiwide-screen television receiver" which can provide a horizontally extended image on the screen without image distortion by reducing the rate of overscanning in a horizontal direction by increasing the horizontal picture size of a screen of a cathode ray tube.
In such a semiwide-screen television receiver, a 12.8.times.9 AR semiwide picture or 12.times.9 AR normal picture is selectively displayed in response to an input picture size signal. When selecting the semiwide picture, i.e., a so-called plus picture, a strict adjustment of the picture size is required. However, if the television signal is pushed or pulled during its transmission, propagation, or reception, the horizontal blanking intervals may appear on the left and right corners of the screen. Also, for the semiwide screen case, the detection of the left and right boundaries of the picture is used for optimally displaying the image signal as wide as possible in both left and right directions, while in the normal picture case it is used for detecting the optimum boundaries of the left and right edge portions.
For to a normal television broadcasting signal, the left and right boundaries of the received image signal are cut with a sufficient margin, and thus the image signal is uniformly displayed on a 4.times.3 AR screen, causing the data at the left and right sides of the received image signal to be lost. Accordingly, in order to solve this problem, a technique has recently been developed, whereby the image signal is displayed on the 12.8.times.9 AR screen by detecting the left and right boundaries where the image signal exists, without uniformly cutting the left and right boundaries of the image signal.
Referring to FIG. 4, a conventional technique for detecting the left and right boundaries of the image signal is explained.
In order to detect the left and right boundaries of the image signal, five check points are determined on each of 37th and 230th horizontal scanning lines, respectively. At the respective check points, it is checked whether or not the luminance signal level of the image signal is higher than a predetermined value, and the checked values are accumulated for 600 fields.
Specifically, if the luminance signal value of the image signal is higher than the predetermined value at each check point, "+1" is accumulated, while if not, "-1" is accumulated. For instance, if the luminance signal having a level over the predetermined level is inputted for 600 fields at the check point 1, the accumulated value will be "+1200" since the check is performed twice per field. The accumulated value for 600 fields at each check point will be in the range of -1200 .about.+1200.
The accumulated value in the range of -1200.about.+1200 is converted into a percentage in the range of 0.about.100%. If the accumulated percentage at each check point is low, it is determined that no significant image signal exists at the corresponding point, while if the accumulated percentage is high, it is determined that a significant image signal exists at the corresponding point.
Specifically, if the accumulated percentage at the first check point is less than 3% and that at the second check point is less than 40% (i.e., case 1), it is determined that no significant image signal exists at the first check point (which is the furthest choice for the left boundry of the image signal). If the accumulated percentage at the fifth check point is less than 3%, that at the fourth check point is more than 40%, and that at the third check point is more than 40% (i.e., case 2), it is determined that no significant image signal exists at the fifth check point (which is the furthest choice for the right boundary of the image signal). If the case 1 or case 2 is produced, the left and right boundaries of the image signal are defined at the second and fourth check points.
On the contrary, if the accumulated percentages at the first, second, third, fourth, and fifth check points are more than 40%, it is determined that significant image signals exist at the first and second check points, and the left and right boundaries of the image signal are defined at the first and fifth check points.
However, according to the conventional detecting technique, since the left and right boundaries of the entire picture are determined by checking only the 37th and 230th horizontal scanning lines for each field, an accurate boundary detection cannot be achieved with respect to the middle portion of the picture. Further, since the values accumulated for 600 fields at five check points are used to calculate a mean value per field, it is difficult to obtain an accurate boundary value.